Recently, techniques for forming capacitance elements comprising ferroelectric films having spontaneous polarization on a semiconductor integrated circuit has been eagerly developed in order to realize nonvolatile RAM (random access memory) which enables low operating voltage, high-speed writing and reading compared to conventional RAMs. In this trend, a group of materials called Bi-layered ferroelectric are developed to be used for such a capacitance insulating film. One of the problems of conventional ferroelectric nonvolatile memories is property deterioration of the ferroelectric film caused by repeated polarization inversion when SrNi.sub.2 Ta.sub.2 O.sub.9, one of the Bi-layered ferroelectric is used. A Bi-layered ferroelectric is a substance having a crystal structure where relatively rough-filled (Bi.sub.2 O.sub.2).sup.2+ layers and pseudoperovskite layers are laminated by turns. The pseudoperovskite layers comprise at least one kind of metal element and oxygen, and they have one or more relatively close-filled virtual perovskite lattices.
Well-known techniques for forming ferroelectric films are, for example, MOD (metal organic deposition), CVD (chemical vapor deposition), and sputtering. Another well-known method for forming a ferroelectric film comprises the following steps: dissolving a metalorganic compound in an organic solvent; coating the solution on a substrate by spincoating; drying the coating and sintering the coating under oxygen atmosphere.
Metalorganic compounds are usually used for forming Bi-layered ferroelectric thin films. Conventionally, however, metalorganic compounds containing the metal elements are used for the respective Bi-layered ferroelectrics. Therefore, the number of different metalorganic compounds used for source materials are increased, and accurate control of the amounts of each metal element which should exist in the film becomes difficult. As a result, it is difficult to form a Bi-layered ferroelectric thin film comprising desired elements with good reproducibility.
When forming a ferroelectric film on a semiconductor integrated circuit, superior step-coating property and processing at a low temperature are required. CVD technique is considered excellent from this viewpoint. When a ferroelectric thin film is formed by CVD technique, the metalorganic compound should be vaporized (gasified) to be fed to the substrate surface. When the metalorganic compound is gasified to be fed to a film-forming chamber, namely, when the metalorganic compound is heated to the vapor pressure required for film-forming, the compound should be prevented from being pyrolyzed or reacting with other substances in the temperature region. On the other hand, the compound should be pyrolyzed rapidly and homogeneously once it reaches the surface of the substrate. The metalorganic compounds which are conventionally used for the Bi-layered ferroelectric (except for Bi itself) do not meet the above requirements, and thus, it is difficult to form thin films of desirable compositions with good reproducibility by using CVD technique.